Ink printing apparatuses can be used for single-color or multicolor printing of a printing substrate, for example of a single sheet or of a web-shaped recording medium made of the most varied materials (paper, for example). The design of such ink printing apparatuses is known; see for example EP 0 788 882 B1. Ink printing apparatuses that operate according to the Drop-on Demand (DoD) principle have a print head or multiple print heads that provide a plurality of printing elements. A printing element thereby comprises an ink channel ending in a nozzle, which ink channel has a piezoactivator. The activators—controlled by a printer controller via control signals developed from control data—excite ink droplets in the direction of the printing substrate, which ink droplets are directed onto the printing substrate in order to apply print dots for a print image there.
The control data with the control signals are obtained in a preparation process from the print data derived from the image to be printed. In this preparation process, with an RIP (Raster Image Processor) the image to be printed is overlaid with a print image raster, wherein a raster point of the print image raster respectively corresponds to a PEL or output pixel. A PEL is the location at which a print dot can be applied. In order to also be able to reproduce grey tones (semitones) in the print image, multiple raster points or PELs can be combined into a print raster cell that is filled with more or fewer print dots depending on the grey value of the print image (WO 94/18786 A1). Before this process runs in the printer, for the printer a print job is developed in which, for example, the type of rastering can be established by adjusting the printer driver options.
In an inkjet printing apparatus, the ink that is used is adapted to the print head in terms of its physical/chemical composition; for example, the ink is adapted in terms of its viscosity. Given low print utilization, not all printing elements of the print head are activated in the printing process. Many printing elements have downtimes, with the consequence that the ink in the ink channels of these printing elements is not moved. Due to the effect of the evaporation out of the nozzle opening, the danger exists that the viscosity of the ink is then altered. This has the consequence that the ink in the ink channel can then no longer move optimally and exit from the nozzle. In extreme cases, the ink in the ink channel dries completely and clogs the ink channel, such that a printing with this nozzle is no longer possible.
A drying of the ink in the printing elements of a print head in their print pauses represents a problem that can be prevented in that a flushing medium (for example ink or cleaning fluid) is flushed through all nozzles of the print head within a predetermined cycle. This flushing cycle can be set corresponding to the print utilization.
The drying of the ink in the nozzles can also be prevented in that printing occurs from all nozzles within a predetermined cycle (refresh measure). This cycle can be set corresponding to the print utilization. Individual points can thereby be applied in unprinted regions of the printing substrate, or print dot lines can be printed between print pages. These methods can lead to disruptions in the print image, in addition to unnecessary ink consumption and additional wear of the print heads. A corresponding refresh measure for an ink printing apparatus is described in US 2012/0 262 510 A1.
Furthermore, from DE 697 36 991 T2 (EP 0 788 882 B1) it is known to remedy the difficulties in the ejection of ink droplets that are caused by alteration of the viscosity of the ink in the nozzles, in that before or after a printing process the piezoelectric activators of the printing elements are respectively set into vibration (also called a prefire measure or meniscus oscillations) such that no ink droplets are ejected, but the ink in the ink channels and nozzles is stirred. It can thereby be achieved that the ink situated in the nozzle openings mixes with the ink located inside the ink channel, such that in the printing operation the ink droplets can again be generated under normal conditions.
From EP 1 795 356 A1, during printing pauses of printing elements of a print head it is known to insert vibration oscillations for these printing elements to avoid the drying of the ink. A printing pause for a printing element is provided if no ink droplets should be ejected with this printing element in the printing operation, thus if what are known as “zero pixels” are present. Via the printer controller, the print data are examined as to whether a multitude of such “zero pixels” follow one another for the printing element. If this is the case, one or more vibration oscillations are triggered. The triggering can be controlled via a print clock pulse.
DE 10 2012 110 187 A1 describes a method to execute an interruption in a printing interruption of an ink printing system in which the respective print image is generated by nozzles of the print head from image points arranged like a raster. Given triggering of a printing interruption, the feed velocity of the printing substrate is reduced from the velocity in the printing operation to a predetermined velocity in a slow-down ramp, and is accelerated again to print velocity in an acceleration ramp after the printing interruption. With the aid of a sensor, print clock pulses are generated from the feed of the printing substrate, which print clock pulses are supplied to a printer controller. Given the occurrence of a print clock pulse during the ramps, the printer controller induces a vibration cycle at nozzles of the print head that do not eject ink droplets.